For an integer m ≥ 2, let ℤ/mℤ be the set of all residue classes mod m. For S ⊆ ℤ/mℤ and \(\bar{n}\in\mathbb{Z}/m\mathbb{Z},\,R_{S}(\bar{n})\) is defined as the number of solutions to the equation \(\bar{n}=\bar{s}+\bar{s^{\prime}}\) with an unordered pair \((\bar{s},\bar{s^{\prime}})\in S^{2}\) and \(\bar{s}\ne\bar{s^{\prime}}\) . In this paper, the author determines the structures of sets A and B such that A ⋃ B = ℤ/mℤ, \(A\;\cap\;B=\bar{k}\mathbb{Z}\) and \(R_{A}(\bar{n})=R_{B}(\bar{n})\) for all \(\bar{n}\in\mathbb{Z}/m\mathbb{Z}\) , where k is an integer.